Your Data Might Still Be Recoverable (But Don't Count On It!)

Since, as you commented, your goal is to ensure that no data from the Windows system can ever be recovered:

No. Nothing you could have done with the Ubuntu installer utility can completely eradicate all possible informational trace of previous data on your disk.

You can, however, probably accomplish this, and if you wish, you can probably accomplish this with the Ubuntu live CD. (You cannot accomplish it from within a running operating system installed on the drive, like your newly installed Ubuntu system.)

To render all data on a disk permanently unrecoverable, you need to overwrite every byte on the disk. This takes a moderately long time (up to several hours, or days for disks that are really big and also really slow).

According to some respected computer engineers and data recovery specialists, to be entirely sure you're destroying all the data, you have to overwrite it with unpredictably random bytes. (On the other hand, my understanding is that some equally learned folks think that's not necessary.)

If you're really interested in doing this, see:

• How can I securely erase a hard drive?

  But remember that some of the information in some or all of those answers might be wrong.

• How can I reliably erase all information on a hard drive?

  Answers there could be wrong too, but they're more likely to have useful comments pointing out that something is wrong, because this is one of the sorts of question that really engages the Security.SE community.

But is that really what's important?

The potential recoverability of data from your old Windows system is entirely irrelevant to all of the following:

• Your Ubuntu system's stability.

• Your Ubuntu system's security.

  (Malware on your old Windows system cannot cause itself to be run. The configuration that gave it power can no longer be attained by booting to it, unless you deliberately recovered the system to the point of being able to run, which probably would not succeed. Even if it did, though, anyone who can perform low-level data recovery of old partitions from previous OS installations that are no longer in the partition table, can also do whatever they want to your computer. In order to perform such a recovery, you see, someone already has to have full access to read and write to any part of your hard disk and to cause your computer to perform any action or run any software. If someone wanted to take control of your computer and/or install unwanted software on it, recovering your old Windows system would be one of the least efficient and most ineffective ways imaginable to do that.)

• Your Ubuntu system's performance.

• The amount of disk space available to your Ubuntu system.

  (If data are recoverable from the old OS, that just means that your Ubuntu system hasn't gotten around to storing its own data in all the same places on the disk that contained the old data. The physical presence of remnants of the old data does not make any difference at all to the speed with which any part of the hard drive can be accessed for [over]writing new data.)

• The stability, security, performance, and disk space available to any future operating system installed on your computer, even if it is a Windows operating system. For the same reasons as explained above, any malware on you previous Windows system still has no mechanism to affect the state of a subsequent Windows OS that is written over it.

Futhermore, assuming the Ubuntu installation worked properly (which is something we can check, see below), you will not be accidentally booting into the old operating system. To get it to even start booting you'd probably have to try hard. If you got it to boot all the way, that would be truly impressive.

Just Installing Ubuntu Over the Old System Seems Reasonable for Most Purposes

This following paragraph pertains to law, but I am not a lawyer, this is not legal advice, and relevant law probably varies from one place to another. Please consider this section to be my personal thoughts. I am not even pretending to be able to tell you what is and is not legal.

If your goal is to make sure you are not infringing copyright law by having an unlicensed copy of Windows, or to make sure you fulfill your end of an end user license agreement that says you must do something like "destroy all copies of the software," it is hard for me to imagine that installing Ubuntu over it, so that the partition table no longer contains any partition entries for your old system, would not fulfill that requirement. I have never heard of anyone interpreting a requirement to "destroy all copies" of a copyrighted work to require that they be destroyed beyond the possibility of even partial recovery. Right now, assuming your Ubuntu installation worked the way it should have (which you can check, see below), your situation is similar to the situation of someone who destroys a paper letter by ripping it up into a thousand pieces, burns a third of them, and then scatters the other two thirds between two separate trash cans in their house that get emptied at different times.

Then again, I don't know everything. That's not legal advice, and the ripping-paper analogy is a technical illustration that may very well be legally irrelevant. If you need legal advice, you should consult with an attorney who practices law in your jurisdiction.

Sometimes You Really Do Have to Destroy All the Data Completely

If there is information in your old Windows system that must be kept secret, for example if you are ethically or legally obligated to ensure that it is not disclosed, then it is a good idea to scramble the disk completely (as discussed in the two questions linked above).

If there is information in your old Windows system that is so sensitive that one or more people could plausibly die as a result of its disclosure, or that something comparably bad could result, then you should scramble the disk completely. Then, you should physically destroy the disk. The reason for this is that, even if the likelihood is small, it is possible that you could make a mistake (or that your software tools could have a bug) that would cause some data to survive. I am not familiar with what techniques are recommended for physically destroying hard disks, but some government agencies and some businesses do actually have protocols for that sort of thing, some of which can be read by interested members of the public.

I am guessing your situation is not like either of those two. This section is included for completeness only (and in case you were wondering under what circumstances I recommend making sure all data on a disk are completely destroyed).

Peace of Mind Is An Acceptable Reason to Securely Erase a Disk

It takes a bit of time to read and absorb information about secure erasure techniques. But deciding what technique or utility to use is ultimately not too difficult, and most hard drives these days don't take overwhelmingly long to overwrite.

Therefore, if it makes you feel better to do it, do it!

• I don't know how well this works compared to other options, as it's been some time since I read up on securely erasing disks. But many hobbyists and small businesses that resell PC's use Darik's Boot And Nuke. If you really want to securely erase your disk, you might want to use that.

Of course, this will overwrite your new Ubuntu system, too. But you just installed it, so you probably haven't customized it much yet. You could probably reinstall it without too much hassle.

It is almost certainly not necessary for you to securely erase your disk, but it's kinda cool, so if you feel like it, go ahead!

You Should Check Your Partition Table(s) to Make Sure Ubuntu Installed the Way You Wanted

Without checking your partition table, you don't know for sure if the Windows partitions themselves were really wiped out. If they weren't wiped out, your Windows system might be fully accessible and you might even end up booting to it again (though this is still not extremely likely, assuming it's not booting now and there's no menu option for it).

It's a good idea to do this. Here's how:

1. Boot your Ubuntu system if it's not already booted, and log on if you don't have it set to log you on automatically.

2. Open a Terminal window by pressing Ctrl+Alt+T.

3. Run this command:

   sudo parted -l
   

   (If you want to run sudo fdisk -l instead, as in fabricator4's answer, that's fine. For this purpose, choosing between them mostly comes down to personal preference.)

4. You'll probably be prompted for your password. This is the same password that you would use to log in, install updates, or unlock the screen. It is the password you set up when you installed Ubuntu.

   • As you enter it, no placeholder characters (like *) appear. That's by design. Just type the password in and press Enter.

5. If you know how to read the output of parted or fdisk, you will be able to see if there are any partitions still existing that belong to your old Windows system.

   Otherwise, as fabricator4 said, you can make sure there are no NTFS partitions (since Windows uses NTFS partitions for its system drives, and by default for all drives).

   Or, you can copy all the text from the Terminal, edit your question, paste it in, and then comment on this answer so I know you did so. I (or someone else) can then look at the output and tell you what each existing partition is for and whether or not there are any partitions that belong to a Windows system or that are likely to have existed before you installed Ubuntu.

I believe checking your partition table to make sure there are no Windows partitions left over is quite sufficient, and I think most other experienced users would agree. Please do not hesitate to ask if you want help interpreting the output of parted or fdisk.